'76 Bronco C-4 Rebuild - Ford Muscle Forums : Ford Muscle Cars Tech Forum
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post #1 of 18 (permalink) Old 01-02-2017, 05:20 PM Thread Starter
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'76 Bronco C-4 Rebuild

Was taking some time to get into gear when shifting from neutral to drive and over 100,000 miles so decided to rebuild.

This is the first time I attempted to rebuild a transmission, but I heard the C-4 was one of the easiest, so I gave it a try. Replaced seals, intermediate band, and clutches even though band and clutches looked very good. Fluid was completely clear with nothing at the bottom of the pan.

Results: Reverse and the 1-2 shift are perfect, but 2-3 is erratic. Usually has a large flare.
If I shift manually between 1st and 2nd good result.
Starting in 2nd manually good result.
Going manually from 2nd to 3rd - large flare. If I let up on the throttle no flare but it takes some time to get into 3rd. Once in third it drives normally.

I did some research and found out that most common cause is improper direct clutch clearance. But before going back to that I found out that the modulator spool valve can be installed backwards.

Would that cause these symptoms or would that cause bad 1-2 shift also?
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post #2 of 18 (permalink) Old 01-03-2017, 12:16 AM
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Re: '76 Bronco C-4 Rebuild

What were you High/reverse clutch clearances ? What modulator did you use ? Where is the modulator adjustment set at. What length pin was used ? What preload on the pin?
Good vacuum supply @ the modulator ?
What was the trans end-float set at ?
Were the clutch drum piston seals replaced as well?
How did it air test on assembly?
What was done with the valvebody?

Everyone says they are easy
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post #3 of 18 (permalink) Old 01-03-2017, 10:03 AM Thread Starter
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Re: '76 Bronco C-4 Rebuild

Thanks for the reply.

Since the original was working well except for taking time to go from neutral to drive or neutral to reverse, and all the internals looked like new, the bottom of the pan and fluid completely clean, I figured that it was mainly a seal problem.

I don't remember clutch clearances (which I now regret).

I used the new modulator that came with the rebuild kit-green stripe, and tried turning the adjustment both ways with no real change.
I tried putting back the original one with same result as the new one.

I used the original pin don't know length.
Have no idea about preload on pin, repair manual (Haynes) didn't mention it. I don't remember any spring going in before modulator spool valve,
Checked vacuum at modulator - same as manifold around 18 lbs at idle.

Don't remember end float but was careful to put thrust washers and shims in their original
positions.

Yes clutch drum piston seals were replaced.

Did air test with good results.

Just carefully cleaned valve body with no changes.

Would putting in spool valve backwards affect both 1-2 and 2-3 shift?
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post #4 of 18 (permalink) Old 01-05-2017, 02:40 AM
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Re: '76 Bronco C-4 Rebuild

You MIGHT be able to tidy up the flare with the modulator . I can't say if the valve backwards would cause that , I've never done it .
If you're unsure check it . There is no spring below the modulator spool, the spring is inside the modulator can. Loosen the mount bracket and feel if the modulator is springy as it is near right in,that is the preload. The manuals tell you the absolute basics only, not how to actually build these things and set up correct first go. That takes experience
To set a modulator correctly is done by weight on a scale to a set value. A normal setting would be APPROX 3-4 turns in

Where the clutch clearances actually checked and within specs . That is a big one that can cause .

Another thing that might help is tighten up the intermediate band adjustment to one turn out
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post #5 of 18 (permalink) Old 01-05-2017, 10:05 AM Thread Starter
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Re: '76 Bronco C-4 Rebuild

OK Thanks Gregaust.

Got that idea about spool valve in backwards from several forums.

I checked clutch clearances but don't remember the numbers.

Taking the C-4 out again, especially with the heavy awkward transfer case attached, is not much fun

Will give your suggestions a try and report back..

I guess the C-4 is easy compared to others but not the first time someone tries it.
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post #6 of 18 (permalink) Old 01-06-2017, 01:52 AM
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Re: '76 Bronco C-4 Rebuild

Out of Curiosity, what servo ??
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post #7 of 18 (permalink) Old 01-06-2017, 09:13 AM Thread Starter
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Re: '76 Bronco C-4 Rebuild - Progress

I am the original owner and tranny was never touched.
So whatever came stock on '76 Bronco is the servo that's on it.
I just took a look and there are a bunch of numbers and a large "H" on the cover.

Last edited by umc2762; 02-03-2017 at 09:07 AM.
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post #8 of 18 (permalink) Old 01-07-2017, 12:18 AM
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Re: '76 Bronco C-4 Rebuild

The H is the I'D . That is a good one
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post #9 of 18 (permalink) Old 02-03-2017, 09:17 AM Thread Starter
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Re: '76 Bronco C-4 Rebuild - Progress

Finally got around to adjusting the intermediate band.
Rechecked Haynes manual and found out my initial adjustment was 1 3/4 turns out.
Readjusted to 1 turn out as suggested and result was very bad.
Truck would not move unless engine RPM went way up and then immediately stopped once accelerator was released. 1 1/4 out was a little better.
Finally at adjustment of 1 3/8 out the 2-3 flare is much improved.
Will now play around with the modulator and report back.
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post #10 of 18 (permalink) Old 02-03-2017, 11:20 PM
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Re: '76 Bronco C-4 Rebuild - Progress

Your front band is coarse thread??? Or fine thread ??
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post #11 of 18 (permalink) Old 02-04-2017, 08:40 AM Thread Starter
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Re: '76 Bronco C-4 Rebuild - Progress

I'll crawl under, take a look, and let you know.
Thanks for your interest.
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post #12 of 18 (permalink) Old 02-04-2017, 09:58 PM
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Re: '76 Bronco C-4 Rebuild - Progress

If it is fine thread that would make a difference . One turn out on a coarse thread is very common.

Truck would not move unless engine RPM went way up and then immediately stopped once accelerator was released.

So you saying here ? With the band at 1 turn out the truck had trouble moving in D in first gear ? The band unless obviously tight should have no effect on taking off .
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post #13 of 18 (permalink) Old 02-05-2017, 03:30 PM Thread Starter
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Re: '76 Bronco C-4 Rebuild - Progress

The front (intermediate) adjusting threads are coarse just like the rear.

I originally had it adjusted to 1 3/4 turns out after 10 lb torque.
I re-torqued to 10 lbs and turned out one turn as you suggested.
Then if I put it in drive or reverse it wouldn't move unless high RPMs and suddenly stopped if I let off the accelerator.
Also noticed that if in neutral I could not push the truck either forward or backward,

So I backed it out to 1 1/4 turns - was then able to move in reverse or drive but when driving at about 30 mph it would suddenly shift back to second without any change in accelerator position.

Now it is 1 3/8 out and the flare is much less and it stays in drive like it is supposed to.
When I have time I'll try adjusting the modulator 1/2 turn at a time.

Last edited by umc2762; 02-06-2017 at 08:48 PM.
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post #14 of 18 (permalink) Old 02-06-2017, 02:31 AM
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Re: '76 Bronco C-4 Rebuild - Progress

Some weird stuff happening here
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post #15 of 18 (permalink) Old 02-06-2017, 08:47 PM Thread Starter
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Re: '76 Bronco C-4 Rebuild - Progress

Just seems as though band isn't releasing as it should (or dragging) at 1 turn out, and 1 3/8 turns out lets it release completely right?
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